The invention relates to an electronic trip device comprising analog measuring means for measuring electrical signals, analog-to-digital conversion means having an input connected to the analog measuring means, digital processing means connected to the conversion means and performing protection functions, and offset correction means.
Numerous offset correction devices are known, in particular in measurement of AC electrical signals. In U.S. Pat. No. 5,060,166, for example, the digital processing circuit of a measuring device uses pair of samples which are 90xc2x0 apart to perform offset correction digitally. This type of correction is complex in so far as it requires the sampling period to be linked to the frequency of the signal to be measured and four successive sample values to be stored in memory.
In other known devices, each measurement sample is associated to an offset measurement, by periodic zero resetting, at the sampling frequency, of the input of the analog measuring channel, upstream from the analog-to-digital converter. For each sample, the processing circuit subtracts the value of the measured offset from the value of the sample. This type of correction slows conversion down as, for each sample, the analog-to-digital converter has to convert both the sample and the associated offset value.
The object of the invention is to achieve an offset correction more particularly suited for use in an electronic trip device and not presenting the drawbacks of known correction devices.
According to the invention, this object is achieved by the fact that the offset correction means comprise digital filtering means of the high-pass type, in the digital processing means on output from the conversion means, and means for initialization of the digital filtering means performing, during an initialization phase, grounding of the input of the conversion means, storing of the output signal from the conversion means and zero resetting of the corresponding output value of the digital filtering means.
According to a development of the invention the digital filtering means perform computation of an output value Si for each sample Ei supplied by the conversion means, according to the equation:
Si=Sixe2x88x921xe2x88x92xcex1Sixe2x88x921+Eixe2x88x92Eixe2x88x921
in which i corresponds to the rank of the sample considered, xcex1 is a constant, which is a function of a sampling period of the signals to be measured and of a cutoff frequency of the digital filtering means, S0=0 and E0 is the value of the output signal from the conversion means obtained when initialization is performed.
The trip device can comprise a plurality of analog measuring channels whose outputs are connected to a multiplexer, connected to conversion means common to all the measuring channels, the digital processing means controlling the multiplexer to reset the input of the conversion means to zero during the initialization phase.